Low polarization sensitivity is critical for a number of Earth science applications, including the measurement of ocean color and ozone from satellite sensors. For example, several National Research Council Decadal Survey science objectives are based on spectrometric measurements over wide spectral ranges and large spatial extents. The upwelling Earth radiation is polarized, with the spatial and spectral polarization texture dependent on variations in the solar scattering geometry, the aerosol, cloud, and molecular content of the line of sight column, and the time dependence of multi-scale weather patterns. While this presents a rich harvest of information for a polarimetric sensor, it simultaneously produces a confounding overlay on the purely photometric assessment of spatio-spectral information, due to innate polarizance in spectrally diverse optical trains. Polarization management is a way of life for designers of spectrometric sensors; without it, the few percent polarization differences from place to place could mask significant differences in ocean color and aerosol loading, for instance hiding toxic algal blooms harboring Vibrio cholerae, or obscuring rich fishing grounds.
Polarization control places serious constraints on the optical designs of sensors, particularly for wide field of view imaging spectrometers. In order to control polarization, low angles of incidence on mirrors and filters have been required. In addition, polarization control has necessitated stringent requirements concerning the optical properties of mirrors, filters, and anti-reflection coatings on system optical elements. A particularly challenging problem is presented by imaging spectrometers, since the diffraction grating used in such instruments is strongly polarizing.
The mitigation of radiometric errors introduced by variable polarization in an incident beam can be achieved by introducing a polarization control element early in the optical train, to produce a known and controllable polarization state in the light seen by the downstream optics. For example, an element that transforms input light into a quasi-unpolarized beam can be provided. In one approach, spatial averaging or recombination is achieved using birefringent crystal devices. Implementations that include birefringent crystals present challenges as a result of the inherent beam deviation that results in such systems. In particular, the beam separation produced by crossed pairs of birefringent material results in four separate polarized images. These images, each uniquely polarized, have a projected separation in object space, resulting in image degradation. For example, for beams that have a projected separation of 0.4 degrees in object space, the resulting ground resolution for a spectrometer at geosynchronous orbit is 12 kilometers. Moreover, the separation of the beams in object space results in an increased point spread function (i.e., a large image spot). Depolarization devices that average in the spectral domain employ color filters. In such devices, polarization varies with wavelength. Because of this wavelength dependent polarization characteristic, depolarizers that average in the spectral domain cannot be used in spectrometers.